Because of its high sensitivity and specificity for detecting a wide range of cancers in oncology, the positron emission tomography (PET) system is becoming very popular for diagnostic study. Coincident detection in the PET system provides the projection sampling which can be reconstructed to yield tomographic images. The primary advantage of PET is its ability to quantify the metabolism activity inside the body.
FIG. 1 shows a conventional PET system with the coincidence circuitry. The PET system utilizes coincident detection of a pair of photons generated by the annihilation of the positron and electron. It includes hundreds of detectors arranged in a ring shape and positioned around an object being tested. Every two detectors are connected by the coincidence circuitry. When two photons are detected at the same time (<12 ns), it is called a coincidence event. The incident direction of photons is the line that connects these two detectors. Many coincidence events can be obtained after scanning for a long time. The whole distribution of emitting sources can be computed and obtained by image reconstruction methods, such as filter backprojection (FBP) and maximum likelihood expectation maximization (MLEM).
Currently, the spatial resolution of a PET system is limited by the high 511-keV photon energy, non-colinearity of the annihilation photons, positron range, and detector technology. A typical small animal PET system provides an absolute sensitivity of approximately 4% and an intrinsic resolution of approximately 1.6 to 1.8 mm (Tsui and Wang 2005).
High-resolution small animal single photon emission computed tomography (SPECT) imaging is possible through the use of pinhole collimation at a much lower cost than small animal PET. There is no theoretic limit of the achievable spatial resolution, it is possible to achieve approximately 1 mm of spatial resolution. The pinhole SPECT requires a heavy collimated detector to rotate around the object and a small misalignment in the setup can generate artifact. Note that to improve the sensitivity, multiple pinholes collimator can be employed. Artefacts can be arisen from overlapping projections in multi-pinhole reconstruction for such a conventional SPECT.
So far 18FDG (fluoro-deoxyglucose) is the most commonly used pharmaceutical for PET study. However, the absorption of FDG is mainly due to the absorption of glucose for the metabolic process and is not organ specific. Furthermore, the short half-life of 18F limits its usage when a long observation is needed.
In recent years, there is a growing interest in the use of non-pure positron emitters. The use of non-pure positron emitters as an alternate to 18F for PET study is gaining popularity in diagnostic or therapeutic radiopharmaceuticals due to their longer half-lives and target specific properties (Herzog et al 2002). The long half-life is advantageous for developing radiochemical syntheses and allows the tracing of slow biochemical processes.
Of particular interest are 38K, 52m Mn, 60Cu, 94 mTc, and 124I. These isotopes are not pure positron emitters; high energy gamma rays (called associated gamma rays) are emitted simultaneously with the positron that can be scattered down to the primary energy window and give rise to random coincidence events. These detected photons are not angularly correlated and therefore contain no information regarding the location of their events. This downgrades the PET performance. The isotropic property of the associated gamma rays causes the events to be evenly distributed across the entire sinogram and contributes only a low frequency background to the reconstructed image.
Robinson et al (2004) claimed that non-pure emitters are not suitable for 3D mode due to the potentially large increase in the observed scatter fraction expected, in “Performance of a block detector PET scanner in imaging non-pure positron emitters—modelling and experimental validation with 124I”, Phys. Med. Biol. 49 5505-28. Schueller et al (2003) investigated the problem of third gamma in PET and concluded that no benefit comes from the third gamma, in “Addressing the third gamma problem in PET”, IEEE Trans. Nucl. Sci. 50 50-2.
Guerra et al (2000) invented a PET-SPECT system. The PET-SPECT system is not stationary, but requires 180° rotations of gantry to collect all the data. In the disclosure of U.S. Pat. No. 6,303,935, John C. Engdahl et al invented a combination PET/single photon (SPECT or planar) nuclear imaging system. As shown in FIG. 2, the system utilizes a pair of dedicated PET detectors 212a-212b and at least one dedicated single photon detectors 214a-214b mounted on a single gantry 210. The PET detectors 212a-212b perform only high energy PET imaging, while single photon detectors 214a-214b perform only low single photon imaging. Simultaneous PET/single photon imaging studies can be carried by the single system. The photon detectors also may be removable and mountable on a separate, dedicated single photon imaging gantry.
Kacperski et al (2004) also proposed a PET system utilizing the coincident detection of 3γ decays from positron annihilation. By the law of energy and momentum conservation, the source position can be determined. However, the accuracy of source positioning is greatly dependent upon the energy resolution of the detectors and the 3γ decay is a rare event which is about 2 orders less than regular 2γ decays.
There are some examples of the important non-pure positron emitters. 76Br (bromine-76) is used for the investigation in molecular imaging (Beattie et al 2003). 86Y (yttrium-86) is used for dose estimation in patient therapies with 90Y-labeled radiopharmaceuticals (Buchholz et al 2003). 82Rb allows the assessment of absolute myocardial perfusion as well as coronary flow reserve (Fakhri et al 2005). The positron emitter 94mTc can be used to improve quantification of tracers currently labeled with 99mTc (Barker et al 2001). 124I PET is utilized in comparative studies in which diagnostic or therapeutic radiopharmaceuticals labeled with the 123I or 131I (Herzog et al 2002). The long half-life of these non-pure positron emitters is advantageous for developing radiochemical syntheses that allows the tracing of slow biochemical processes which cannot be adequately examined by the commonly used short-lived positron emitters.